Origin of the unusually strong and selective binding of vanadium by polyamidoximes in seawater

Abstract

Amidoxime-functionalized polymeric adsorbents are the current state-of-the-art materials for collecting uranium (U) from seawater. However, marine tests show that vanadium (V) is preferentially extracted over U and many other cations. Herein, we report a complementary and comprehensive investigation integrating ab initio simulations with thermochemical titrations and XAFS spectroscopy to understand the unusually strong and selective binding of V by polyamidoximes. While the open-chain amidoxime functionalities do not bind V, the cyclic imide-dioxime group of the adsorbent forms a peculiar non-oxido V⁵⁺ complex, exhibiting the highest stability constant value ever observed for the V⁵⁺ species. XAFS analysis of adsorbents following deployment in environmental seawater confirms V binding solely by the imide-dioximes. Our fundamental findings offer not only guidance for future optimization of selectivity in amidoxime-based sorbent materials, but may also afford insight to understanding the extensive accumulation of V in some marine organisms.

Fig. 1

Critical metals in seawater and extraction systems. a Estimated ratio of the amounts of selected critical metals in the oceans to the terrestrial reserves. b Adsorption kinetics of vanadium, uranium, iron, and copper by typical polyamidoxime adsorbents after 56 days of contact with seawater (Sequim Bay, USA) in flow-through columns. c Schematic depiction of a small subsection of the polyamidoxime fiber. d Cartoon representation of vanabin2 with the highlighted amino acid residues participating in vanadium binding^,

Fig. 2

DFT-optimized geometries of V(V) complexes. a V(V) complexes with glutaroimide-dioxime (H₃IDO). b V(V) complexes with acetamidoxime (HAO). Color legend: V(V), cyan; O, red; N, navy blue; C, gray; H, white

Fig. 3

Potentiometric and calorimetric titrations along with V(V) speciation diagrams under seawater conditions. a Potentiometric titration for the complexation of vanadium with H₃IDO at 25 °C and I = 0.5 M (NaCl); observed (diamonds) and calculated (−) pH (left axis) with corresponding speciation (right axis); titration conditions: V _o = 20.2 mL, [V] = 1.01 mM, [H₃IDO] = 10.3 mM, [H] = 0.0129 M, titrant: 0.100 M HCl. b Calorimetric titration thermogram showing heat flow rate after each injection. c Cumulative observed (diamonds) and calculated (−) heats (left axis) with corresponding speciation (right axis) per injection; titration conditions: V _o = 0.750 mL, [V] = 1.0 mM, [H₃IDO] = 4.16 mM, [H] = 3.32 mM; titrant: 0.020 M HCl; 5 μL per injection. d Speciation of vanadium under seawater conditions with H₃IDO and e HAO ligands; [V] = 3.6 × 10⁻⁸ M, [H₃IDO] = 0.5 M, [HAO] = 0.5 M. Structures of prevalent species at the seawater pH are visualized for clarity

Fig. 4

EXAFS spectra and magnitude of the Fourier transform of small molecule standards, solution samples, and seawater-contacted adsorbents. Solid samples: a Na₃VO_4(s). b Na[VO₂(HIDO)]_(cr). c Na[V(IDO)₂]_(cr). Solution samples: d Na₃VO₄. e [VO₂(IDO)]²⁻. f [V(IDO)₂]⁻. Seawater-contacted adsorbents: g AF-1, h AI-8. All samples show the data as a solid black line and fit as a dotted red line